1. Field of the Invention
The present invention relates to a laser diode, and more particularly, to a self-pulsating laser diode smoothly generating a stable ultra-short pulse using active mode-locking by input of an external radio frequency (RF) among mode-locking phenomena in a laser diode not having a non-linear section such as a saturable absorber.
2. Discussion of Related Art
An optical pulse is used for various purposes, such as clock reproduction by a re-amplifying, re-shaping and re-timing (3R) reproducer in optical communication, a light source in optical communication, a light source for optical sampling, a carrier for RF communication, and so on. In particular, when an optical pulse is generated using a semiconductor laser diode, a device using the same has advantages in size and economical efficiency. For this reason, much research on pulse laser diodes has been conducted.
A physical mechanism causing a laser diode to emit a pulse may be Q-switching, gain switching, mode beating, or mode locking. Since an RF signal is used in Q-switching or gain switching, there are limitations due to the speed of an electronic device and the response speed of a laser diode. Meanwhile, mode beating is generally implemented by a laser diode having a plurality of distributed feedback (DFB) sections. However, a very precise process is required to form these DFB sections.
Mode locking is roughly classified into active mode-locking and passive mode-locking. According to active mode-locking, phase locking, i.e., fixing of a phase difference between modes, is performed by an external RF signal. According to passive mode-locking, a nonlinear section such as a saturable absorber is input into the laser diode to perform phase locking.
However, according to active mode-locking and passive mode-locking, the width of a variable frequency is small, and therefore the manufacturing process is difficult. For example, when a reference frequency is 40 GHz, the width of a variable frequency is less than 1 GHz (2.5%). Therefore, it is difficult to adjust a frequency to a reference frequency by applying current, etc., after forming a device. As a result, a device must be precisely fabricated.
Meanwhile, a laser diode comprising a DFB section, a phase control section, and a gain section generates optical pulses using mode beating occurring between compound cavity modes.
More specifically, when a current equal to or greater than a threshold current is applied to the DFB section, the DFB section operates as a single-mode laser to emit light to the phase control section and the gain section. The emitted light passes through the phase control section and the gain section, and is reflected by an as-cleaved facet, i.e., a cut plane of the end of the gain section, to return back to the DFB section. Due to such reflection, the entire device does not operate in a single mode but oscillates in two modes. The device generates an optical pulse using beating between the two modes.
When an optical pulse is generated by beating between the two modes, the DFB section may be used as just a reflector by applying a current less than a threshold current to the DFB section. In this case, a new pulse is generated.
Since the DFB section serves simply as a reflector, the pulse generation is performed in a single-cavity laser diode in which reflection at an as-cleaved facet and reflection at a DFB reflector constitute a resonator.
In such a structure, physical mechanisms causing pulse emission are Q-switching and mode locking, which is seen as quasi passive mode-locking whereby a fixed number of resonator modes contribute to generating an ultra-short pulse, in consideration of a change in frequency according to device length.
In this connection, such a passive mode-locking phenomenon is observed in a distributed Bragg reflector (DBR) laser diode also. As reported, a normal waveguide structure does not smoothly generate a pulse. However, when a waveguide is formed in an almost square shape, a pulse is smoothly generated by passive mode-locking.
Currently, interaction between nonlinear modes, such as four wave mixing (FWM), is drawing attention as a reason causing passive mode-locking. The waveguide structure having the square cross-section also increases only a light restriction coefficient of the waveguide to increase the nonlinear effect. Here, the light restriction coefficient indicates how much the waveguide restricts light.
However, it is significantly difficult to fabricate a waveguide having a shape close to a square in the case of a laser diode using a DBR.